One of NSW's most senior lawyers warns government plans to bring in tougher penalties to combat lethal 'coward punches' will not solve the problem.

Former president of the NSW Law Society, Maurie Stack, said the notion that bringing in new laws to lock offenders up in jail for longer would stop the unprovoked violent attacks was "complete nonsense".

The NSW government plans to bring in a new offence of 'unlawful assault causing death' with a maximum penalty of 20 years jail. It removes a legal requirement to prove 'coward punch' assailants knew the punch would be fatal to secure a conviction.

The move comes after outrage over the four years non-parole sentence given to the one-punch killer of Sydney teenager Thomas Kelly last year.

"I'm sick of reading in the press that the solution to the coward punch is to increase the period of imprisonment for the offender," said Mr Stack, Chairman of Stacks/The Law Firm.

"Does anyone seriously believe that an aggressive young male fuelled by alcohol and steroids picks up the phone and rings his lawyer to check the maximum prison sentence before attacking some innocent passerby? The idea is complete nonsense and at odds with much of the criminology research that has been published.

"This proposal is peddled by those in the media who attract an audience by appealing to our base instinct for revenge and politicians afraid of an electoral backlash and of a huge media campaign from the liquor industry if they address the real problem by taking steps to reduce the consumption of liquor by young people.

"In the long run, the most expensive and least effective solution is to lock up all the offenders for longer. It costs the community more than $100,000 per offender per year. It may not sound like much until you recognise that the Carr government increased our prison population from about 7,500 to 10,000, which is about double the rate in Victoria. That's more than a billion dollars a year not available for hospitals, schools and roads in NSW.

"We also have to recognise as a community that the problem will never totally go away. All we can do is take steps to reduce the problem; effective steps like those taken in Newcastle and some other rural areas. We need to mount a public campaign to ridicule macho bullies."

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